Tree harvesting machines with saw chain and guidebar of types corresponding to those used with hand-held chain saws are known from among others the patents U.S. Pat. No. 3,991,799, 4,116,250, 4,161,200, 4,219,059 and 4,552,191. In these the guidebar is attached to a mounting base rotatable around the driving shaft from the motor to the drive sprocket for the saw chain. The guidebar and saw chain are advanced towards the tree to be felled by a hydraulic cylinder acting on the mounting base or a shaft connected to it.
In tree harvesting machines the saw chain is highly stressed, runs at high speeds and wears rapidly. This leads to expensive downtime for changing or tensioning of the saw chain, and to the possibility that the saw chain becomes so slack that it will jump off the guidebar.
It is important for the safety that the saw chain is tensioned with appropriate force, since too low force might allow the links of the chain to tilt and leave the guiding groove of the guidebar, and too high force will impair the lubrication of the saw chain and wear the rivets too soon.
In prior art, the tensioning force in the saw chain is normally adjusted by loosening the bolts holding the guidebar, followed by moving the guidebar lengthwise by a screw mechanism, and finally retightening said bolts. This does not ensure proper control of the tensioning force since there is no means for measuring the force, and since the force is affected by wear and temperature rise of the saw chain in use.
This adjustment procedure requires that the machine is stopped, and that the operator or another skilled person performs the adjustment with appropriate tools. The loss of production time and the need to leave the operator's cab make it likely that the adjustment is seldom done except when the saw chain or the guidebar has to be changed, which is too infrequent.